This invention relates to a method and device for controlling the tracking in a rotary head type magnetic recording/reproducing apparatus.
One example of a tracking control method in a magnetic recording and reproducing apparatus using a rotary magnetic head, such as a VTR, etc. is disclosed in JP-A-53-89407.
In this conventional tracking control method, at a recording mode, video signals are recorded on a video track in such a way that the vertical synchronizing signals of the video signals to be recorded and a disk tach pulses (hereinafter referred to as DTP) generated in synchronism with the rotation of a magnetic head are controlled by a servo control device used when recording the video signals so that they are in a predetermined phase relation. At a reproduction mode, tracking control is performed so that the magnetic head correctly scans the video track by controlling the rotating speed of the magnetic head so that fixed is the time corresponding to the phase difference between the vertical synchronizing signals, separated from the video signals reproduced from the video track, and the DTP's.
However, the servo control device at a recording mode, mentioned above has an essential problem that the rotating speed and rotating phase of the magnetic head of recording video signals vary for several reasons and so the vertical synchronizing signal is not record on a fixed position, which results in the variation of the recording position thereof; the above several reasons include the variations or changes of the load torque of a motor which rotates the magnetic head, power supply voltage in a servo circuit system, environmental temperature and humidity, elapse of time, etc.
Therefore, if the recorded tape is tracking-controlled by the conventional technique mentioned above, the time corresponding to the phase difference between the vertical synchronizing signals and the DTP's will include a variation error as well as a true tracking error; this variation error corresponds to the variation during recording and is irrelevant to the tracking error. Then, the control device acts to fix the tracking error including the variation error to a certain value so that a new tracking error will be produced. Further, if the variation during recording is too large, the control system is out of its operation range, thereby making the tracking control impossible and the system unstable.
Moreover, the variation during recording includes a stationary position deviation depending upon the rotation phase deviation of the magnetic head. This position deviation also is generally too large to ignore, and its value is different for each device, thereby making the reproduction by the exchange of the device very difficult.
There has been proposed another tracking control method in JP-A-57-66559. In this method, index signals created synchronously with the DTP signal are recorded so as to be inserted during the vertical blanking interval of the video signal, and using the index signals the tracking control is performed during reproducing. This method can remove the influence from the variation and deviation at a recording mode. However, since the number of the index signals which can be recorded for detecting the tracking errors is essentially limited, the sampling rate of detecting the tracking errors can not be sufficiently enhanced, thereby making difficult the tracking control at a higher precision or accuracy.